Lifestyle Choices Linked to Weight Gain in Young Females

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Explain to interested patients that the study authors suggested that modifying activities outside of school or work may slow weight gain.

Point out that observational studies cannot establish causality.

BOSTON, July 9 -- The way teenage girls and young women spend their leisure time has a strong influence on whether they gain weight, a longitudinal study showed.

More time spent on the Internet, fewer hours of sleep, and alcohol consumption were all associated with increases in body mass index (P<0.05 for all), Catherine Berkey, Sc.D., of Harvard, and colleagues reported online in the Journal of Pediatrics.

Those who fell into each of these categories, particularly those older than 18, were expected to gain about four extra pounds a year, the researchers said.

"Although the estimated effects were small, they may accumulate over time and become clinically important as these behaviors are maintained, or more likely magnified, as girls approach adulthood," they said.

"Older girls may benefit from replacing recreational Internet time with sleep and by avoiding alcohol," they said.

The results came from the Growing Up Today Study, which included a survey of 5,036 daughters of participants in the nationwide Nurse's Health Study II.

Each participant -- ages 14 to 21 at the time -- filled out a questionnaire about leisure-time activities and sleep in the past year and self-reported height and weight for two consecutive years.

Almost all of the participants (95%) were white. Most of them were normal weight, but 12.7% were overweight, 4.8% were obese, and 4.7% were very lean.

Most (87%) of the young women were spending one hour or more a week on recreational Internet use, only 31.3% were sleeping eight or more hours a night, 36% drank coffee, and 35% drank alcohol.

Starting at age 18, participants drank more alcohol than coffee.

Internet usage, sleep, and alcohol and coffee consumption were significantly correlated with each other (P<0.0001 for all), with one exception. Coffee consumption was not significantly correlated with Internet time or weight gain (P=0.54).

As the number of hours spent on the Internet per week increased, so did BMI (+0.045 kg/m2 per hour/day, P<0.05), although controlling for physical activity, TV watching, and video game play attenuated the association.

The weight gain was likely the result of decreased energy expenditure associated with increased sedentary time, the researchers said.

BMI also went up as the amount of sleep per night decreased (P<0.05), even after controlling for potential confounders.

Young women 18 and older who slept five or fewer hours per night gained 0.322 kg/m2 more per year than those who slept eight hours or more a night (P<0.01).

"Insufficient sleep might result in daytime fatigue and reduced physical activity, with fewer calories expended during the day," the researchers said. Metabolic effects may also influence weight gain, they said.

In participants of all ages, BMI also increased with greater alcohol consumption (P<0.05), although the association was not significant in those 18 and older.

The researchers found no evidence that drinking coffee leads to weight gain.

The authors acknowledged several limitations of the study, including the inability of longitudinal studies to establish causality, the self-reported data, potential residual confounding, the omission of persons who were pregnant or smokers, and possible reporting errors.

Also, they said, the cohort was not representative of young U.S. females, and the results may not be generalizable to a more diverse population.

Even so, the researchers concluded, "if future studies confirm our findings â€¦ then adolescents could safely and inexpensively use this information to manage their body weight."

The study was supported by the NIH, the Breast Cancer Research Foundation, the Economic Research Service of the U.S. Department of Agriculture, the Boston Obesity Nutrition Research Center, and a Prevention Research Center grant from the CDC. The authors declared no conflicts of interest.

Reviewed by Zalman S. Agus, MD Emeritus Professor University of Pennsylvania School of Medicine

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